The present invention relates to abrasion resistant papers such as overlays or decor sheets useful in decorative laminates, and in particular, to abrasion resistant papers incorporating spacer or separator particles to minimize the amount of damage to highly polished caul plates caused by contact of the caul plates with abrasion resistant materials during the lamination process. The invention also relates to a process for manufacturing such abrasion resistant papers.
Decorative laminates are conventionally produced by stacking and curing under heat and pressure a plurality of layers of paper impregnated with a synthetic thermosetting resin. In normal practice the assembly from the bottom up, consists of three to eight core sheets made from phenolic resin impregnated kraft paper, above which lies a pattern or decor sheet impregnated with melamine resin; on top of the decor sheet is provided an overlay sheet which, in the laminate, is almost transparent and provides protection for the pattern sheet.
The overlay sheet is almost invariably used when the decor or pattern sheet has a surface printing in order to protect the printing from abrasive wear. The overlay sheet is usually a high quality alpha cellulose paper of about 20-30 pounds ream weight that is also impregnated with melamine-formaldehyde resin in a manner similar to that used for the decor sheet, except that a greater amount of resin per unit weight of paper is used. The individual sheets are stacked in the manner indicated above.
It is well known that the addition of small, hard abrasion resistant particles (also referred to as xe2x80x9cgritxe2x80x9d) to the overlay paper, or to resin mixtures which coat the impregnated decor sheet, can enhance the abrasion resistance of high-pressure laminates. Alumina has been used to give wear resistance of 400 to 600 cycles. However, abrasion resistant particles tend to scratch and cause significant damage to highly polished caul plates used during the lamination process for producing both high pressure and low pressure laminates. Caul plates scratched or otherwise damaged through contact with abrasion resistant materials must either be resurfaced or replaced at a significant cost. The estimated cost to resurface the caul plate is currently approximately $0.01 to 0.02/ft2.
One of the conventional methods for producing abrasion resistant laminates without damaging the caul plates involves the use of release paper which provides a physical barrier separating the abrasive grit particles from the caul plates. The use of release paper is undesirable from a cost perspective. The release paper costs approximately $0.02 to $0.05/ft2. Various attempts have been made to produce an abrasion resistant overlay or decor sheet which could be formed into a decorative laminate without damaging the expensive, highly polished caul plates and did not require the use of release paper. U.S. Pat. No. 4,971,855 to Lex, et al. discloses the use of extremely small (less than 9 microns) abrasion resistant particles in the production of a wear resistant, glossy laminate which does not result in rapid destruction of the caul plates during lamination. U.S. Pat. No. 5,545,476 to O""Dell, et al. discloses a wear and abrasion resistant glossy laminate having a thick protective coating incorporating pre-cured thermoset resin particles of up to 250 microns which protect the caul plates from the smaller abrasion resistant particles in the overlayer coating.
These previous methods for producing an abrasion resistant laminate without causing rapid destruction of the expensive caul plates are not without potential disadvantages. Small particle size grit, while preventing rapid destruction of the polished caul plates, still results in scratches on the caul plate surface where the grit contacts the caul plate. Furthermore, larger particle size grit is preferred as the larger particles, in general, provide better abrasion resistance. The effectiveness of pre-cured resin particles as spacers or separator particles can vary depending a number of variables, such as, the degree of cure, particle size distribution, particle shape and particle distribution within the resin matrix. Therefore, it would be desirable to be able to produce high and low pressure laminates exhibiting improved abrasion resistance associated with the use of larger size grit while providing protection for the expensive caul plates from the abrasive grit particles.
This invention relates to wear resistant papers and particularly to wear resistant papers which are useful in forming decorative laminates. The invention also relates to methods for manufacturing these wear resistant papers. In accordance with the invention, microspheres are incorporated into a wear resistant sheet to minimize caul plate damage associated with the use of abrasion resistant particles. Abrasion resistant particles or grit are added to the sheet to impart wear resistant properties to the paper. The microspheres preferably are larger than the abrasion resistant particles and therefore function as spacer or separator particles providing protection for the caul plates from the abrasion resistant particles in the wear resistant paper. It has been found that the larger microspheres prevent excessive contact between the abrasion resistant particles and the expensive caul plates during subsequent lamination operations using the wear resistant paper. Accordingly, the invention provides for a wear resistant laminate that does not adversely affect the life of the caul plates. A principal use of wear resistant paper made in accordance with the invention is in decorative laminates of the type used for flooring and similar products. The microspheres and abrasion resistant particles are both applied on the paper machine in a cost effective, continuous process.
In accordance with a preferred embodiment of the invention, glass microspheres are intermixed with the fiber as a part of the paper matrix and abrasion resistant grit particles are separately applied to the paper web during the continuous manufacture of wear resistant paper on a commercial scale. In one embodiment, the microspheres are added to the fiber, for example by feeding a slurry of the microspheres continuously to a pulp stream that feeds the primary head box, and the mixture of microspheres and fiber is formed into a sheet on the wet end of a paper machine. In a second embodiment, the microspheres are sprayed onto the web from a fluidized bed. In another embodiment, the microspheres are mixed with water and applied to a fibrous web on the wet end of a paper machine using a coater such as a curtain or slot coater. In still another embodiment, the microsphere slurry is mixed with fiber and applied to the paper machine from a primary or secondary head box.
The term xe2x80x9cwet endxe2x80x9d as used herein refers to any location on the paper machine in the paper manufacturing process prior to the dryer can and particularly includes the addition of microspheres to the pulp stream feeding the head box, to the pulp in the fan box, in the beaters, or in a storage chest, spraying microspheres onto the web at any location prior to the dryer can, addition of the microspheres to the white water and applying a mixture of the microspheres and fiber as a surface coating by means of a secondary headbox or a slot orifice coater such as a curtain coater prior to the dryer can, e.g., between the dandy roll and the wet press, as well as a combination of these points prior to the dryer can, for example addition at the beaters and by application from a spray unit.
The term xe2x80x9cslot coater or orifice slot coaterxe2x80x9d as used herein includes coaters in which the coating passes through an orifice and forms a curtain which falls on the web and coaters in which the coating is extruded through a slot where it forms a bead which contacts the web.